Induction crucible furnaces are optimal for carbon-neutral melting of various metals. In a crucible-less induction furnace, a ceramic crucible is situated within a cylindrical copper coil. The material in the crucible is exposed to an electromagnetic field generated by the current flowing through the copper coil, inducing eddy currents in the metal. These eddy currents cause ohmic losses that heat and eventually melt the metal.
These induction crucible furnaces achieve efficiencies of over 80% for ferrous materials and over 70% for highly conductive materials like copper or aluminum, thanks to the high conductivity of the copper coil and the low-loss return of the electromagnetic field through transformer steel yokes.
An alternative design for an induction furnace is the channel furnace principle. In this design, one or more channel inductors are flanged to a main furnace vessel. The heating of the liquid melt follows the principle of a short-circuited transformer: a copper coil forms the primary winding, and a channel of liquid metal forms the short-circuited secondary winding. The short-circuit current flowing in this channel causes ohmic losses, which heat the metal. This electrical principle achieves efficiency 10-15% higher than the crucible furnace principle but requires the furnace to avoid complete drainage, limiting alloy and operational flexibility.
Additionally, crucible furnaces allow for higher power capacities. Crucible furnaces for iron and steel, with over 20 MW power and melting capacities exceeding 40 tons per hour, are successfully in operation.
Minimizing Energy Consumption in Induction FurnacesEven though induction furnaces can operate CO2-free with renewable electricity, foundries strive to minimize energy consumption for sustainability and cost-efficiency, particularly with rising energy costs. Melting iron or aluminum requires about 500-560 kWh/ton, making energy costs a significant factor in casting and semi-finished product manufacturing.
Three key factors influence energy savings during the melting process in foundries:
1.
System Dimensioning and Power Supply Contracts: Current power supply contracts typically include charges for capacity, energy, and reactive power. Reducing costs associated with capacity charges involves ensuring consistent energy use from the grid. To avoid peak loads that the energy supplier must provision and charge for, furnace systems should be sized to cover the planned liquid metal demand, with temporary peaks managed through additional production times. Monitoring systems that reduce consumption when reaching peak load levels can help. Induction furnace systems with pulse-width modulated IGBT converters maintain a constant power factor even at partial loads, providing good electrical efficiency for parallel resonant converters. Series resonant converters also maintain a constant power factor but incur higher losses due to uncompensated furnace currents.
2.
Operational Practices for Melting Furnaces: Practical operational techniques can save significant energy, similar to efficient driving in cars. Optimization starts with the selection and processing of charge materials.
3.
Analysis of Digital Melt Process Data: Modern induction furnace systems come equipped with PLC controls and process computers that store critical melt process data for each charge. A comprehensive charge documentation includes the energy consumed, melt temperature, and material feed times. Reviewing these data can identify irregularities and optimize the melting process.
Practical Furnace Operating MethodsEnergy savings in furnace operation can be substantial through strategic practices, including:
Material Preparation: Clean charge materials to avoid impurities like sand and rust that reduce energy efficiency.
Maximizing Pack Density: High pack density reduces energy consumption by improving coupling with the induction field.
Optimizing Carburizing: Adding carburizing agents at the beginning of the melt saves energy compared to adding them to the molten bath later.
Using Full Power: Operating the furnace at maximum available power reduces energy consumption by shortening melt times and reducing thermal losses.
Melting without Sump: Medium-frequency technology allows efficient melting of small materials without a sump, reducing energy needs by improving electromagnetic coupling.
Covering the Furnace: Keeping the furnace covered retains heat and reduces energy loss.
Adjusting Exhaust Systems: Tailoring exhaust rates to actual needs prevents unnecessary energy loss.
Avoiding Overheating: Preventing unnecessary overheating saves energy.
Enhancing Efficiency through Data-Driven MethodsEfficient operation in foundries heavily depends on data analysis and adherence to standard operating procedures (SOPs). Implementation of ideal melting processes involves continuous monitoring and adjustment to minimize energy wastage. Transparent real-time data displays, such as dashboards, can motivate operators by showing the economic impact of their actions and the quality of charge materials. Additionally, artificial intelligence can assist in analyzing melt process data to identify patterns and suggest optimization strategies for material charging and furnace operation.
Achieving Energy BalanceIntegrating furnace data with Manufacturing Execution Systems (MES) and Enterprise Resource Planning (ERP) systems can synchronize production with periods of lower energy prices, contributing to grid stability and increased use of renewable energy. This strategic alignment can provide significant refunds from grid operators for participating in energy balancing, enhancing sustainability and reducing the carbon footprint.
References:
- Trauzeddel, D. (2018). Special Applications of Inductive Melting and Casting Technology. Vulkan Verlag.
- Dötsch, E. (2019). Inductive Melting and Holding. Vulkan Verlag.
- Donsbach, F.; Schmitz, W.; Trauzeddel, D. (2018). OTTO JUNKER Handbook: Safe and Energy-Efficient Melting in MF Crucible Furnace. Self-published.
- Donsbach, F.; Renftle, G.; Niklaus, S. (2021). Melting of Low-Density Scrap in Medium-Frequency Induction Crucible Furnaces. Technical Article.
- OTTO JUNKER Academy.
- Photos: OTTO JUNKER and INDUGA Archives.
- Init Group.
Authors:- Frank Donsbach, OTTO JUNKER GmbH, Germany
- Matias Mohedano Rodriguez, OTTO JUNKER GmbH, Germany
- Ulrich Nordt, OTTO JUNKER GmbH, Germany
- Peter Koldig Hansen, Init Inuatek A/S, Denmark
